5 Flashcards
Who discovered that DNA is the genetic material?
Multiple experiments revealed this, including those by Griffith, Avery, and Hershey-Chase.
What did Griffith’s experiment show?
Genetic transformation—non-virulent bacteria became virulent when mixed with heat-killed virulent bacteria.
What molecule did Avery, MacLeod, and McCarty identify as responsible for transformation?
DNA.
What was the key conclusion of the Hershey-Chase experiment?
DNA, not protein, carries genetic information in phages.
What method did Hershey and Chase use to distinguish DNA and proteins?
Radioactive labeling—32P for DNA and 35S for protein.
What organism was used in the Hershey-Chase experiment?
Bacteriophage T2 and E. coli.
What was the role of bacteriophages in DNA experiments?
They were used to infect bacteria and trace genetic material.
Why did Avery’s experiment support DNA as genetic material?
Transformation did not occur when DNA was destroyed.
What does ‘transformation’ mean in genetics?
The uptake of external DNA by a cell, altering its genotype.
What type of experiment was used by Griffith?
A bacterial transformation experiment with pneumococcus strains.
What is a bacteriophage?
A virus that infects bacteria.
Why did Hershey and Chase use sulfur and phosphorus isotopes?
Sulfur is found in proteins, not DNA; phosphorus is found in DNA, not proteins.
What happened to the radioactive phosphorus in the Hershey-Chase experiment?
It entered the bacterial cells, showing DNA was injected.
Why was the discovery that DNA was genetic material significant?
It identified the molecule responsible for heredity.
What conclusion did Griffith draw from his experiment?
A ‘transforming principle’ from dead cells could make live cells virulent.
What is DNA made of?
Nucleotides composed of a sugar, phosphate, and nitrogenous base.
What are the two types of nitrogenous bases?
Purines (A, G) and pyrimidines (C, T).
What does Chargaff’s rule state?
The amount of A = T and G = C in DNA.
Who produced the X-ray diffraction images of DNA?
Rosalind Franklin.
What did Franklin’s X-ray images suggest?
DNA is a double helix with 10 nucleotides per turn.
Who built the first correct model of DNA?
Watson and Crick.
How are the DNA strands oriented?
Antiparallel—one 5′ to 3′, the other 3′ to 5′.
What type of helix is DNA?
Right-handed double helix.
What holds the two strands of DNA together?
Hydrogen bonds between complementary bases.
What forces stabilize DNA besides hydrogen bonds?
van der Waals forces between adjacent bases.
Where are the sugar-phosphate backbones located in DNA?
On the outside of the helix.
What do the major and minor grooves in DNA allow?
Protein binding to specific base pair sequences.
What does antiparallel mean?
The two strands of DNA run in opposite directions.
What is exposed in the grooves of DNA?
The outer edges of the nitrogenous bases.
Why is complementary base pairing important?
It allows accurate DNA replication and mutation detection.
What is the distance between base pairs in the DNA helix?
Approximately 0.34 nanometers (nm).
How many base pairs are in one full turn of the DNA helix?
10 base pairs.
Why does A pair with T and G with C?
Because of consistent hydrogen bonding and molecular size compatibility.
What chemical groups are on the 5′ and 3′ ends of DNA?
5′ has a phosphate group; 3′ has a hydroxyl group.
What is meant by DNA having a uniform diameter?
Purine-pyrimidine pairing ensures even spacing between strands.
Which base pairs are stronger: A-T or G-C?
G-C (3 hydrogen bonds vs. 2 for A-T).
Right-handed twist of DNA
It affects how proteins interact with the double helix.
Property of DNA supporting its function
Stable, easily replicated, and stores complex information.
DNA’s structure stability
Hydrogen bonds and hydrophobic base stacking.
Functions of major and minor grooves
Sites for protein binding and regulatory control.
Nucleotide
The monomer of DNA, consisting of a sugar, phosphate, and nitrogen base.
Sugar in DNA
Deoxyribose.
Difference between purines and pyrimidines
Purines have two rings (A, G); pyrimidines have one (C, T).
Linking of nucleotides in DNA strand
By phosphodiester bonds between the 3′ OH and 5′ phosphate.
Antiparallel DNA strands
For proper base pairing and enzymatic function in replication.
Basic process of DNA replication
The DNA double helix unwinds, and each strand serves as a template for a new strand.
Correct model of DNA replication
Semiconservative replication.
Experiment proving semiconservative replication
The Meselson-Stahl experiment using nitrogen isotopes.
Semiconservative meaning
Each new DNA molecule has one old and one new strand.
Enzyme unwinding DNA helix
Helicase.
Enzyme synthesizing new DNA strands
DNA polymerase.
Enzyme adding a primer to DNA
Primase.
Composition of primers
RNA.
Prevention of strand re-annealing during replication
Single-strand binding proteins (SSBs).
Direction DNA polymerase builds new strand
5′ to 3′ direction.
Leading strand
The strand synthesized continuously in the direction of the replication fork.
Lagging strand
The strand synthesized in fragments (Okazaki fragments) opposite to the replication fork.
Enzyme connecting Okazaki fragments
DNA ligase.
Role of topoisomerase
Relieves supercoiling ahead of the replication fork.
Replication fork
The Y-shaped region where DNA is being unwound and replicated.
Initiation of DNA replication
Specific sequences called origins of replication.
Replication bubbles
Regions where DNA has unwound and replication is occurring in both directions.
Direction DNA polymerase reads template strand
3′ to 5′ direction.
Okazaki fragments
Short DNA segments synthesized on the lagging strand.
Reason for lagging strand forming fragments
DNA polymerase can only add nucleotides in the 5′ to 3′ direction.
Fate of RNA primers after synthesis
They are removed and replaced with DNA by DNA polymerase I.
Enzyme proofreading new DNA strand
DNA polymerase.
High fidelity in DNA replication
Proofreading by DNA polymerase and mismatch repair mechanisms.
Speed of DNA polymerase synthesis
Several hundred nucleotides per second.
Direction of DNA replication
The antiparallel nature of DNA strands.
Role of sliding clamp protein
It keeps DNA polymerase attached to the DNA strand.
End of linear chromosomes during replication
The end-replication problem occurs; telomeres prevent information loss.
Telomere
A repetitive DNA sequence at chromosome ends that protects genes.
Enzyme extending telomeres
Telomerase.
Activity of telomerase
In germ cells, stem cells, and cancer cells.
DNA repair systems
They maintain genetic integrity and prevent mutations.
Types of DNA damage
Depurination, deamination, oxidation, and replication errors.
Proofreading in DNA replication
The removal of incorrectly paired bases by DNA polymerase during synthesis.
Mismatch repair
A mechanism that corrects errors missed by DNA polymerase after replication.
Mismatch repair system recognition
In bacteria, it uses methylation patterns; in eukaryotes, it’s less well understood.
Enzyme that cuts mismatched DNA strand
A nuclease.
Post-mismatch cut process
DNA polymerase fills the gap and DNA ligase seals it.
Excision repair
A repair process that removes damaged bases or nucleotides.
Types of excision repair
Base excision repair and nucleotide excision repair.
Base excision repair
Removal of a single altered base.
Nucleotide excision repair
Removal of a segment of DNA containing a bulky lesion (like thymine dimers).
Cause of thymine dimers
UV radiation.
Syndrome from nucleotide excision repair defect
Xeroderma pigmentosum.
Danger of xeroderma pigmentosum
Cells cannot repair UV damage, leading to skin cancers.
Role of DNA ligase in repair
It seals nicks in the DNA backbone after repair synthesis.
Depurination
The loss of a purine base (A or G) from the DNA molecule.
Deamination
The removal of an amino group from a base, often converting cytosine to uracil.
Uracil in DNA damage
Uracil is normally found in RNA, not DNA; its presence can cause mutations.
Enzyme that removes damaged bases in base excision repair
DNA glycosylase.
Enzyme that cuts DNA backbone after base removal
AP endonuclease.
Filling in missing nucleotide during base excision repair
DNA polymerase.
Sealing the DNA strand after repair
DNA ligase.
Damage repaired by nucleotide excision repair
Bulky distortions like thymine dimers and chemical adducts.
Proteins involved in nucleotide excision repair
A group of enzymes including endonucleases and helicases.
Mutation
A permanent change in the DNA sequence.
Effects of mutations on proteins
They can alter the amino acid sequence, possibly affecting function.
Are all mutations harmful?
No, some are neutral or even beneficial.
Somatic mutation
A mutation that occurs in body cells and is not inherited.
Germline mutation
A mutation in reproductive cells that can be passed to offspring.
Common DNA damage caused by UV light
Thymine dimers.
PCR
Polymerase Chain Reaction.
Purpose of PCR
To amplify specific DNA sequences.
Inventor of PCR
Kary Mullis in 1983.
Three main steps of PCR
Denaturation, annealing, and extension.
Denaturation in PCR
DNA is heated (~95°C) to separate the strands.
Annealing in PCR
Primers bind to the DNA template at lower temperatures (~50-65°C).
Extension in PCR
DNA polymerase adds nucleotides to build new strands (~72°C).
Type of DNA polymerase used in PCR
Taq polymerase (from Thermus aquaticus).
Reason for using Taq polymerase
It is heat-stable and can withstand the high temperatures of PCR.
Primers in PCR
Short single-stranded DNA sequences that flank the target region.
How many copies of DNA are produced after 30 PCR cycles?
Over a billion copies (amplified exponentially).
What is required in a PCR reaction mix?
Template DNA, primers, Taq polymerase, dNTPs, buffer, and Mg²⁺.
What does PCR allow scientists to do?
Analyze, clone, or sequence specific genes from small DNA samples.
Why is PCR used in forensics?
To amplify trace amounts of DNA for identification.
What is the role of magnesium ions (Mg²⁺) in PCR?
They act as cofactors for DNA polymerase activity.
Can RNA be used in PCR?
No, but it can be converted to DNA using reverse transcriptase for RT-PCR.
What is RT-PCR?
Reverse transcription PCR; it amplifies DNA made from RNA templates.
How is PCR used in medical diagnostics?
To detect genetic mutations, pathogens, and viral load (e.g., HIV, COVID-19).
What is gel electrophoresis?
A method to separate DNA fragments by size using an electric field.
Why are primers specific in PCR?
They determine which DNA region will be amplified.
What is real-time PCR (qPCR)?
A technique to quantify DNA in real time during amplification.
How can PCR help in cloning genes?
It amplifies specific sequences that can be inserted into plasmids.
What is DNA fingerprinting?
A method using PCR and electrophoresis to identify individuals by DNA profiles.
Why is contamination a problem in PCR?
Trace amounts of DNA can be amplified and give false results.
What is a thermocycler?
A machine that changes temperatures for PCR cycles automatically.
What is molecular cloning?
The process of inserting DNA into a host organism for expression or analysis.
What is a vector in genetic engineering?
A DNA molecule (like a plasmid) used to deliver genetic material into cells.
Why is high specificity important in PCR?
To ensure only the target DNA is amplified.
How does PCR relate to evolution studies?
It can amplify ancient or rare DNA to study evolutionary relationships.
How is DNA replication different from PCR?
DNA replication occurs in cells using complex enzymes; PCR is in vitro and uses Taq polymerase and thermal cycling.
What are the building blocks of DNA?
Nucleotides.
What characteristic of DNA allows it to be copied precisely?
Complementary base pairing.
Why does DNA have to be replicated before cell division?
To ensure each daughter cell receives an identical copy.
What is the role of hydrogen bonds in DNA?
They hold the two strands together and allow separation during replication.
What would happen if DNA polymerase lacked proofreading ability?
Mutation rates would increase due to uncorrected errors.
What is the significance of Chargaff’s rule?
It supports the idea of complementary base pairing in the double helix.
How does semiconservative replication preserve genetic information?
Each daughter molecule receives one parental strand as a template.
Why are telomeres important?
They protect chromosome ends and prevent information loss.
What happens if DNA repair mechanisms fail?
Mutations accumulate, potentially leading to cancer or genetic diseases.
How does UV light damage DNA?
By forming thymine dimers that distort the helix.
What is the significance of the Meselson-Stahl experiment?
It demonstrated that DNA replicates semiconservatively.
What is the role of helicase in replication?
It unwinds the DNA helix at the replication fork.
What would happen without ligase during replication?
Okazaki fragments would not be joined into a continuous strand.
Why are dNTPs essential in PCR and replication?
They are the nucleotide building blocks for new DNA strands.
How does DNA structure explain its ability to be easily replicated and repaired?
Its double-stranded, complementary nature allows for accurate copying and damage recognition.
